In a message dated 07/11/05, charles.r.patton@........ writes:

> I love the discussion and the simple tests for getting to 
> the low friction combinations, but once the hinge friction (which 
> multi-hour swinging certainly qualifies) is significantly below the 
> level you'll use for damping even the longest period pendulum, then I 
> would think other considerations are more important such as the 
> stability and robustness of the hinge against large side forces (read 
> big quake such as those in California are prone to) displacing the hinge 
> point, changing the geometry, and hence the functionality of the 
> pendulum.  

Hi Charles,

       Yes and no. Some hard measurements to suppliment design decisions and 
investigate problems are very welcome. 
       One factor which limits very long period seismometer suspensions is 
non elastic / anelastic effects. No one is surprised to be able to bend a strip 
of say 1/2" x 1/16" mild steel. If you try the same with a strip of hardened 
high carbon steel, it flexes, but then springs back when the bending force is 
released - very nearly, but not quite 100%. And it does so in tiny jumps which 
can be detected using a piezo crystal in good contact with the metal - bending 
mild steel is not a 'smooth' operation - it is the sum of millions of tiny 
steps.

On a pure geometry basis, the hinge point is the means by 
> which the frame moves, moving the pivot point, leaving the bob weight 
> stationary (on a instantaneous basis). Insufficient side friction, and this goes 
> badly awry. Which is why I always perk up when the discussions hinges (pun 
> 

       Rigidity is OK. Damping is OK. If you get ANY friction, you are likely 
in trouble.

These have very large stability (are rigid) against side forces. I was 
especially 
> impressed with the crossed wire discussion a few days ago. The thing that 
> always bothered me about all those flexible hinge types is discerning the 
> actually hinge rotation point/tragetory. Brett Norden has done excellent work 
> figuring out some of them. The question is that if the point of rotation 
> travels, then does it do so in such a way to lead to stability or un-stability 
> 

       See also Re: Crossed wire suspensions - more analysis    Date: 
25/10/05 To PSN 

       Long period swing problems may be related to the design of the 
suspension, the stability and levelling of the apparatus, or to anelastic behavoir of 
the hinge materials. The Australian Folded Pendulums were reported working 
out to 90 sec. using 7 foils. It is more usual these days to use a force balance 
technique to the extend the period electronically. A factor of x10 should be 
relatively easy to achieve.

Rollamites are 
> probably subject to another problem which is dust collection, but the 
> crossed wire 'Rollamite" version would be almost immune, again why I thought that 
> was an interesting suspension. The downside of that suspension would seem to 
> be the orthoganol unstability, i.e., the hinge is relatively rigid in the 
> plane of the rotation, but the orthogonal axis is another hinge with a different 
> period potentially  

       You can design a shield making the system virtually immune to dust. 

       You need to mechanically design your suspension system to cope with 
fairly strong motions and to return to it's original setting after a 
disturbance. If you are in a region subject to severe quakes, it is usual to have a 
'strong motion' sensor. No seismometer is completely indestructable. 

> Oh yes, one other point about points on surfaces, it would seem to me that 
> mixing material is a good idea. I belive I've heard that generally sliding 
> surface bearings are better if the materials are different  I'm not a 
> tribologist, but I'm sure this extends to the application of the ball point rolling on 
> 
  
     Not particularly, so far as I know as applied to seismometers. If you 
use say bare steel, you can get contact welding by compressing / rubbing the 
surfaces together. You may also get stress / frictional activation of a surface 
leading to oxidation / corrosion. Engine and other oils contain 'extreme 
pressure additives'. These may be organic phosphorous compounds and any exposed 
metal surface immediatly reacts giving it a protective surface layer just atoms 
thick. This normally prevents welding.
       There are some particular lubricated plain bearing combinations, like 
bronze on steel, which give a very long life, but this is not applicable to 
seismometers. Bronze is not a very hard alloy, but the surface can work harden 
in the extreme. 

       Regards,

       Chris Chapman
In a message=20=
dated 07/11/05, charles.r.patton@........ writes:



I love the discussion and t=
he simple tests for getting to=20

the low friction combinations, but once the hinge friction (which=20

multi-hour swinging certainly qualifies) is significantly below the=20

level you'll use for damping even the longest period pendulum, then I=20

would think other considerations are more important such as the=20

stability and robustness of the hinge against large side forces (read=20

big quake such as those in California are prone to) displacing the hinge=
=20

point, changing the geometry, and hence the functionality of the=20

pendulum.  



Hi Charles,



       Yes and no. Some hard measurements=20=
to suppliment design decisions and investigate problems are very welcome.=20

       One factor which limits very long p=
eriod seismometer suspensions is non elastic / anelastic effects. No one is=20=
surprised to be able to bend a strip of say 1/2" x 1/16" mild steel. If you=20=
try the same with a strip of hardened high carbon steel, it flexes, but then=
 springs back when the bending force is released - very nearly, but not quit=
e 100%. And it does so in tiny jumps which can be detected using a piezo cry=
stal in good contact with the metal - bending mild steel is not a 'smooth' o=
peration - it is the sum of millions of tiny steps.



On a pure geometry basis, the hinge point is the means by=20

which the frame moves, movi=
ng the pivot point, leaving the bob weight stationary (on a instantaneous ba=
sis). Insufficient side friction, and this goes badly awry. Which is why I a=
lways perk up when the discussions hinges (pun intended) around the Rollamit=
e, crossed leaf hinges, etc. 



       Rigidity is OK. Damping is OK. If y=
ou get ANY friction, you are likely in trouble.



These have very large stability (are rigid) against side forces. I was e=
specially=20

impressed with the crossed=20=
wire discussion a few days ago. The thing that always bothered me about all=20=
those flexible hinge types is discerning the actually hinge rotation point/t=
ragetory. Brett Norden has done excellent work figuring out some of them. Th=
e question is that if the point of rotation travels, then does it do so in s=
uch a way to lead to stability or un-stability swings, i.e., what is the lon=
gest period possible before it might go unstable? 



       See also Re: Crossed wire suspensio=
ns - more analysis    Date: 25/10/05 To PSN=20



       Long period swing problems may be r=
elated to the design of the suspension, the stability and levelling of the a=
pparatus, or to anelastic behavoir of the hinge materials. The Australian Fo=
lded Pendulums were reported working out to 90 sec. using 7 foils. It is mor=
e usual these days to use a force balance technique to the extend the period=
 electronically. A factor of x10 should be relatively easy to achieve.



Rollamites are=20

probably subject to another=
 problem which is dust collection, but the crossed wire 'Rollamite" version=20=
would be almost immune, again why I thought that was an interesting suspensi=
on. The downside of that suspension would seem to be the orthoganol unstabil=
ity, i.e., the hinge is relatively rigid in the plane of the rotation, but t=
he orthogonal axis is another hinge with a different period potentially &nbs=
p;making the seismometer sensitive to another axis 



       You can design a shield making the=20=
system virtually immune to dust.=20



       You need to mechanically design you=
r suspension system to cope with fairly strong motions and to return to it's=
 original setting after a disturbance. If you are in a region subject to sev=
ere quakes, it is usual to have a 'strong motion' sensor. No seismometer is=20=
completely indestructable.=20



Oh yes, one other point abo=
ut points on surfaces, it would seem to me that mixing material is a good id=
ea. I belive I've heard that generally sliding surface bearings are better i=
f the materials are different  I'm not a tribologist, but I'm sure this=
 extends to the application of the ball point rolling on a surface, type bea=
ring, too.

  

     Not particularly, so far as I know as applied t=
o seismometers. If you use say bare steel, you can get contact welding by co=
mpressing / rubbing the surfaces together. You may also get stress / frictio=
nal activation of a surface leading to oxidation / corrosion. Engine and oth=
er oils contain 'extreme pressure additives'. These may be organic phosphoro=
us compounds and any exposed metal surface immediatly reacts giving it a pro=
tective surface layer just atoms thick. This normally prevents welding.

       There are some particular lubricate=
d plain bearing combinations, like bronze on steel, which give a very long l=
ife, but this is not applicable to seismometers. Bronze is not a very hard a=
lloy, but the surface can work harden in the extreme.=20



       Regards,



       Chris Chapman